1. Field of the Invention
The present invention relates to the field of micro electro mechanical systems (MEMS). More particularly, the present invention relates to an electrostatic microactuator referred to as a pendulum motor that is fabricated using surface micromachining techniques.
2. Related Art
Microelectro Mechanical Systems (MEMS) technology has developed rapidly over the past decade due in part to an increased interest in communications systems. MEMS technology used in the fabrication of microstructures includes both bulk and surface micromachining techniques. Bulk micromachining refers to etching the surface of a silicon wafer, while surface micromachining refers to selective etching of mulitlayer thin films on the surface of a silicon wafer.
Microactuators have been developed for use as switches in optical switching applications using current MEMS technology. Electostatically activated microactuators are inherently fast and have low power consumption. In addition, such switches have low insertion loss, reduced crosstalk, and are both wavelength and polarization independent. Furthermore, the size and weight of such optomechanical switches are considerably smaller than previously used optomechanical switches. Batch fabrication reduces cost and provides the means for monolithic integration of large switch matrices on a single semiconductor chip.
A number of electostatically activated microactuators useful as optical switches are currently fabricated on the surface of a silicon wafer using both bulk and surface micromachining techniques. However, some of the materials and processes used in the fabrication of existing microactuators are incompatible with current CMOS technology. Furthermore, the movable portions of these actuators generally exhibit only small displacements, and are capable of movement only in a direction perpendicular to the surface plane of the silicon wafer.
Greater displacement of a moveable member in an electrostatically activated device requires a corresponding increase in the magnitude of an actuating voltage. Higher voltages are of course limited by the breakdown voltage of air or the gas in which the actuator operates. Strict mechanical accuracies during fabrication are also required to produce very smooth surfaces between parts of an electrostatically activated actuator. Otherwise, asperities on electrode surfaces may induce localized electric field emissions or corona discharge at low electric field levels thus further limiting the upper value of applied actuator voltage.
Accordingly, what is needed is a MEMS device that can be fabricated using current CMOS fabrication techniques. What is needed further is a MEMS device that can be fabricated using CMOS technology and can also be used in the field of optical switching. These and other advantages of the present invention not described above are explained in discussions to follow.
Embodiments of the present invention provide a micro electro mechanical system (MEMS) fabricated on a semiconductor wafer using process steps that are compatible with current CMOS technology. The mechanism formed is a pendulum motor having a stator and a moveable cantilever beam. Both the stator and the cantilever beam with one end coupled to a structural layer are formed by etching one or more sacrificial layers deposited on the surface of a structural layer.
A novel step in the fabrication process separates a portion of the cantilever beam from the structural layer by utilizing xenon di-flouride for gas-phase selective etching of a sacrificial layer between the structural layer and the beam. Both the beam and the structure coupling of the beam to the structural layer are flexible, and the uncoupled end of the beam is therefore displaceable from an equilibrium position. A series of electrodes placed on the stator and the beam may be used to create an electrostatic field across a gap separating the beam from the stator. The electrodes are arranged such that the beam may be displaced and held in one or more predetermined positions by the application of a suitable electrostatic field.
Movement of the free end of the beam is in a plane parallel to the surface of the structural layer and may trace either a circular or a non-circular path. Using the beam as a waveguide or as a support for optical fiber, a signal entering at the fixed end of the beam may then be directed to any one of numerous signal receptors positioned along the path of the free end of the beam. It is to be appreciated that optical mirrors may be placed on the surface of the cantilever beam to form an optical switching mechanism.
In one embodiment, what is described is a pendulum motor having a stator and a cantilever beam moveable by an electrostatic force to predetermined positions relative to a supporting structure. A micro electro mechanical mechanism is formed on a semiconductor substrate using process steps that are completely compatible with current CMOS technology. The mechanism consists of a cantilever beam fixed at one end to a structural layer and an electrode arrangement that provides an electrostatic field across a gap between the beam and a stator. A force between the beam and the stator generated by the electrostatic field results in movement of the free end of the beam relative to the stator.
The free end of the beam may then be displaced by a predetermined amount from an equilibrium position and held in place by means of an applied electrostatic field. Movement of the free end of the beam is in a plane parallel to the surface of the support structure and it may follow either a circular or a non-circular path. In one embodiment, the beam is used as a waveguide to form a 1xc3x97N switch, the signal entering the fixed end of the beam and exiting the free end of the beam at one of N receptors depending upon beam displacement. In a second embodiment, a 1xc3x97N switch is formed using attached fiber optics. In a third embodiment, two pendulum motors configured fixed end to fixed end are used to form a one channel blocking Mxc3x97N switch.
More specifically, a first embodiment of the present invention includes releasing portions of a member from a structural layer by etching a sacrificial layer between the member and the structural layer. The released portions of the member may be in the form of a cantilever beam which is moveable with respect to the structural layer. The beam itself may be solid or hollow, and remains attached to the structural layer at a single position by means of a post structure between the beam and the structural layer. In this first embodiment, the unattached end of the beam is in the form of a segment of a disk positioned parallel to the surface of the structural layer and located such that the centerline along the length of the beam bisects the segment and passes through the center point of the disk. Movement of the hammer head shaped end of the beam about the attachment post follows a circular path parallel to the structural layer. An electrostatic field across a gap between one or more electrodes placed to form a stator on the structural layer and one or more electrodes on the beam may be used to generate a force sufficient to displace and hold the unattached end of the beam in a predetermined position relative to the stator.
A second embodiment of the present invention includes releasing portions of a member from a structural layer by etching a sacrificial layer between the member and the structural layer. The released portion of the member is in the form of a cantilever beam which is moveable with respect to the structural layer. The beam itself may be solid or hollow, and one end of the beam is attached at a single position to a supporting structure which is attached to the structural layer. In this second embodiment, the unattached end of the beam is in the form of a torus placed parallel to the surface of the structural layer and located such that the centerline along the length of the beam bisects the torus and passes through the generation point of the torus. Movement of the torus shaped end of the beam about the attachment point follows a non-circular path parallel to the structural layer.
An electrostatic field across a gap between one or more electrodes placed on the stator and one or more electrodes on the beam may be used to generate a force sufficient to displace and hold the unattached end of the beam in a predetermined position relative to the structural layer. In one embodiment of the present invention, a series of electrode elements are arranged in a plane on the surface of the substrate to form a stator and a single electrode is positioned in a parallel plane on the surface of the beam. In a second embodiment, the electrode elements of the stator and the beam are arranged to form a comb type electrode arrangement. In a third embodiment, the electrode elements forming the stator and the beam may be arranged at various angles with respect to the surface of the substrate.
The process steps used in the fabrication of the present invention are compatible with current CMOS process steps. The first step includes the deposition of a structural layer on the surface of a substrate. A second structural layer is then deposited over a sacrificial layer in a pattern to form a first member and a second member that are separated by the sacrificial layer. The first member is then further developed by deposition of silicon nitride over a second sacrificial layer. A crucial and unique step in this process involves selective gas phase etching with xenon di-flouride to separate portions of the first member from the second member leaving the first member attached to the second member at a single position. The first member is in the form of a cantilever beam, the free end of which is moveable. The beam may remain solid or may be etched to form a hollow beam. A series of electrode elements are then positioned on the second member to form a stators and at least one electrode element is positioned on the beam. An electrostatic field across a gap between electrodes on the stator and one or more electrodes on the beam may be used to generate a force sufficient to displace and hold the unattached end of the beam in a predetermined position relative to the stator.